This invention realtes to a fluororesin which possesses flexibility and sufficient mechanical strength and is good in melt processability. The fluororesin is a blend of a crystalline fluororesin and a soft fluororesin which is essentially a graft copolymer obtained by graft polymerization of a fluorine-containing monomer, which gives a crystalline polymer, in the presence of an elastomeric fluoro-polymer.
Various fluororesins are known as functional resins excellent in chemical resistance, weather resistance, heat resistance and so on. Nowadays fluororesins are indispensable materials in many fields of high-technology.
Where good flexibility is required for fluororesins as in the cases of using in hoses, gaskets, seals, cable coverings and roll sheathings it is usual to use fluororubbers. However, processing of fluororubbers requires complicated operations because molding of fluororubber needs to be preceded by kneading of a mixture of raw rubber and additives such as stabilizers, fillers and cross-linking agent and needs to be followed by a cross-linking treatment at a controlled temperature. For the same reason there are limitations on the shapes of articles or parts to be formed of fluororubbers. In some cases it offers another problem that cross-linked fluororubbers can hardly be remelted for further processing.
In view of such inconveniences of fluororubbers there is a strong demand for development of fluororesins which possess flexibility and melt-processability including remeltability.
One way of obtaining a flexible and melt-processable fluororesin is copolymerization. However, improvement in flexibility by copolymerization is usually accompanied by lowering in melting temperature of the fluoro-copolymer so that the copolymer becomes inferior in heat resistance to conventional thermoplastic fluororesins, or the copolymer becomes unsatisfactory in mechanical properties. Another way is polymer blending. In practice, however, there are not many kinds of polymers that are well compatible with fluoro-polymers and serve the plasticizing purpose. Even though a polymer reported to be good in compatibility is used for blending it is often that the important properties of the fluororesin are deteriorated by the blending.
U.S. Pat. No. 4,472,557 discloses fluorine-containing graft copolymers that possess flexibility and melt-processability. The graft copolymers are comprised of a fluorine-containing elastomeric polymer segment and a fluorine-containing crystalline polymer segment, and either of the two types of polymer segments is grafted to the other. The graft polymerization is accomplished by utilizing thermal decomposition of peroxy bonds contained in the "trunk" polymer segment. From a practical point of view, much interests is shown in the graft copolymers using a fluorine-containing elastomeric polymer as the "trunk" segment. In this type of graft copolymers the inferior melt-processability of the fluoroelastomer is compensated for and is remarkably improved by grafting a fluorine-containing crystalline polymer. Furthermore, at temperatures below the melting point of the grafted crystalline polymer the crystalline polymer segments act as physical cross-linking points so that the graft copolymer exhibits physical properties resembling the properties of a chemically cross-linked fluoro-elastomer.
In a graft copolymer of the aforementioned type using a given combination of an elastomeric fluoro-polymer and a crystalline fluoro-polymer the degree of flexibility of the copolymer depends mainly on the molecular weight of the crystalline fluoro-polymer used as the "branch" segments and the number of the branches. An increase in either the molecular weight of the crystalline fluoro-polymer or the number of the crystalline branches leads to an increase in the aforementioned physical cross-linked points and, therefore, brings about enhancement of the modulus of elasticity and hardness of the graft copolymer. Besides, an increase in the physical cross-linking points brings about lowering of melt viscosity and enhancement of melt flow rate of the graft copolymer. That is, in general an increase in the weight percentage of the crystalline fluoro-polymer in the graft copolymer leads to enhancement of both elastic modulus and melt flow rate of the graft copolymer, whereas a decrease in the same component leads to enhancement of flexibility of the graft copolymer and lowering of melt flow rate of the copolymer.
Thus, the degree of flexibility and melt flow rate of the graft copolymer can freely be adjusted by varying the weight ratio of the grafted crystalline fluoro-polymer to the elastomeric fluoro-polymer used as the "trunk" segment. Therefore, for each combination of elastomeric and crystalline fluoro-polymers it is possible to produce graft copolymers of many grades so as to cover a very wide range that extends from a near-fluororubber region to a near-rigid fluororesin region. Accordingly a wide variety of applications are found in the fluorine-containing graft copolymers. However, the need of manufacturing industrial products of so many grades for each type of fluorine-containing graft copolymer offers inconvenience to the manufactures.
The disclosure of U.S. Pat. No. 4,472,557 includes a fluororesin composition obtained by blending 100 parts by weight of polyvinylidene fluoride (PVDF) with 1-100 parts by weight of a soft fluororesin prepared by graft polymerization of vinylidene fluoride (VDF) in the presence of an elastomeric fluoro-polymer having peroxy bonds. The blending has the effect of improving a crystalline PVDF resin particularly in impact resistance and low temperature characteristics.